Monday, November 16, 2009

Harvard materials scientists have come up with what they believe is a new way to model the formation of glasses, a type of amorphous solid that includes common window glass.

Glasses form through the process of vitrification, in which a glass-forming liquid cools and slowly becomes a solid whose molecules, though they've stopped moving, are not permanently locked into a crystal structure. Instead, they're more like a liquid that has merely stopped flowing, though they can continue to move over long stretches of time.

"A glass is permanent, but only over a certain time scale. It's a liquid that just stopped moving, stopped flowing," said David Weitz, Mallinckrodt Professor of Physics and Applied Physics at Harvard's School of Engineering and Applied Sciences (SEAS) and the Department of Physics. "A crystal has a very unique structure, a very ordered structure that repeats itself over and over. A glass never repeats itself. It wants to be a crystal but something is preventing it from being a crystal."

Other than window glass, made from silica or silicon dioxide, Weitz said many sugars are glasses. Honey, for example, is not a glass at room temperature, but as it cools down and solidifies, it becomes a glass.

Scientists like Weitz use models to understand the properties of glasses. Weitz and members of his research group, together with colleagues at Columbia University and the University of North Texas, report in this week's Nature a new wrinkle on an old model that seems to improve how well it mimics the behavior of glass.

The model is a colloidial fluid, a liquid with tiny particles, or colloids, suspended evenly in it. Milk, for example, is a familiar colloidial fluid. Scientists model solidifying glasses using colloids by adding more particles to the fluid. This increases the particles' concentration, making the fluid thicker, and making it flow more slowly. The advantage of this approach to studying glasses directly is size, Weitz said. The colloid particles are 1,000 times bigger than a molecule of a glass and can be observed with a microscope.

"They're big; they're slow. They get slower and slower and slower and slower," Weitz said. "They don't behave like a fluid. They don't behave like a crystal. They behave in many ways like a glass."

The problem with traditional colloids used in these models, however, is that they often rapidly solidify past a certain point, unlike most glasses, which continue to flow ever more slowly as they gradually solidify. Weitz and colleagues created a colloid that behaves more like a glass in that way by using soft, compressible particles in the colloid instead of hard ones. This makes the particles squeeze together as more particles are added, making them flow more slowly, but delaying the point at which it solidifies, giving it a more glasslike behavior.

By varying the colloidal particles' stiffness, researchers can vary the colloidal behavior and improve the model's faithfulness to various glasses.

"There's this wealth of behavior in molecular glass and we never saw this wealth of behavior in colloid particles," Weitz said. "The fact you can visualize things gives you tremendous insight you can't get with molecular glass."

In one sense, our hands define our humanity. Our opposable thumbs and our hands' unique structure allow us to write, paint, and play the piano. Those who lose their hands as a result of accident, conflict or disease often feel they've lost more than mere utility.

A new invention from Tel Aviv University researchers may change that. Prof. Yosi Shacham-Diamand of TAU's Department of Engineering, working with a team of European Union scientists, has successfully wired a state-of-the-art artificial hand to existing nerve endings in the stump of a severed arm. The device, called "SmartHand," resembles — in function, sensitivity and appearance — a real hand.

Robin af Ekenstam of Sweden, the project's first human subject, has not only been able to complete extremely complicated tasks like eating and writing, he reports he is also able to "feel" his fingers once again.

In short, Prof. Shacham-Diamand and his team have seamlessly rewired Ekenstam's mind to his SmartHand. Prof. Shacham-Diamand's contribution to the project, on which TAU collaborated with Sweden's Lund University, is the interface between the body's nerves and the device's electronics. "Perfectly good nerve endings remain at the stem of a severed limb," the researcher says. "Our team is building the interface between the device and the nerves in the arm, connecting cognitive neuroscience with state-of-the-art information technologies."

Prof. Shacham-Diamand runs one of the top labs in the world for nano-bio-interfacing science: the Department of Electrical Engineering — Physical Electronics Lab under the Bernard L. Schwartz Chair for Nano-scale Information Technologies. "Our challenge," remarks Prof. Shacham-Diamand, "was to make an electrode that was not only flexible, but could be implanted in the human body and function properly for at least 20 years."

The artificial SmartHand, built by a team of top European Union scientists, will belong to Ekenstam, the test subject, as long as he wishes. "After only a few training sessions, he is operating the artificial hand as though it's his own," says Prof. Shacham-Diamand. "We've built in tactile sensors too, so the information transfer goes two ways. These allow Ekenstam to do difficult tasks like eating and writing."

Ekenstam told a television interviewer, "I am using muscles which I haven't used for years. I grab something hard, and then I can feel it in the fingertips, which is strange, as I don't have them anymore. It's amazing."

This particular multi-million dollar project focused on hands, but the TAU/EU team could also have built bionic legs to be wired to the brain. The team first chose to build a hand, however, because of its unique challenges. "The fingers in the hand are the most complex appendages we have," Prof. Shacham-Diamand observes. "The brain needs to synchronise the movement of each digit in a very complicated way."

With the help of the TAU team, the SmartHand project was able to integrate recent advances in today's "intelligent" prosthetic hands with all the basic features of a flesh-and-blood hand. Four electric motors and 40 sensors are activated when the SmartHand touches an object, not only replicating the movement of a human hand, but also providing the wearer with a sensation of feeling and touch.

While the prototype looks very "bionic" now, in the future SmartHand scientists plan to equip it with artificial skin that will give the brain even more tactile feedback. The researchers will also study amputees equipped with the SmartHand to understand how to improve the device over time.

When small earthquakes shake the central U.S., citizens often fear the rumbles are signs a big earthquake is coming. Fortunately, new research instead shows that most of these earthquakes are aftershocks of big earthquakes (magnitude 7) in the New Madrid seismic zone that struck the Midwest almost 200 years ago.

The study, conducted by researchers from Northwestern University and the University of Missouri-Columbia, was published in the Nov. 5 issue of the journal Nature.

"This sounds strange at first," said the study's lead author, Seth Stein, the William Deering Professor of Geological Sciences in the Weinberg College of Arts and Sciences at Northwestern. "On the San Andreas fault in California, aftershocks only continue for about 10 years. But in the middle of a continent, they go on much longer."

There is a good reason, explains co-investigator Mian Liu, professor of geological sciences at Missouri. "Aftershocks happen after a big earthquake because the movement on the fault changed the forces in the earth that act on the fault itself and nearby. Aftershocks go on until the fault recovers, which takes much longer in the middle of a continent."

The difference, Stein explains, is that the two sides of the San Andreas fault move past each other at a speed of about one and a half inches in a year -- which is fast on a geologic time scale. This motion "reloads" the fault by swamping the small changes caused by the last big earthquake, so aftershocks are suppressed after about 10 years. The New Madrid faults, however, move more than 100 times more slowly, so it takes hundreds of years to swamp the effects of a big earthquake.

"A number of us had suspected this," Liu said, "because many of the earthquakes we see today in the Midwest have patterns that look like aftershocks. They happen on the faults we think caused the big earthquakes in 1811 and 1812, and they've been getting smaller with time."

To test this idea, Stein and Liu used results from lab experiments on how faults in rocks work to predict that aftershocks would extend much longer on slower moving faults. They then looked at data from faults around the world and found the expected pattern. For example, aftershocks continue today from the magnitude 7.2 Hebgen Lake earthquake that shook Montana, Idaho and Wyoming 50 years ago.

"This makes sense because the Hebgen Lake fault moves faster than the New Madrid faults but slower than the San Andreas," Stein noted. "The observations and theory came together the way we like but don't always get."

Aftershocks go on for long times in other places inside continents, Stein said. It even looks like we see small earthquakes today in the area along Canada's Saint Lawrence valley where a large earthquake occurred in 1663.

The new results will help investigators in both understanding earthquakes in continents and trying to assess earthquake hazards there. "Until now," Liu observed, "we've mostly tried to tell where large earthquakes will happen by looking at where small ones do." That's why many scientists were surprised by the disastrous May 2008 magnitude 7.9 earthquake in Sichuan, China -- a place where there hadn't been many earthquakes in the past few hundred years.

"Predicting big quakes based on small quakes is like the 'Whack-a-mole' game -- you wait for the mole to come up where it went down," Stein said. "But we now know the big earthquakes can pop up somewhere else. Instead of just focusing on where small earthquakes happen, we need to use methods like GPS satellites and computer modeling to look for places where the earth is storing up energy for a large future earthquake. We don't see that in the Midwest today, but we want to keep looking."